1. Field of the Invention
The present invention relates to a method of operating a laser light source for use in various kinds of inspections, measurements, and processes and, more particularly, to a method of operating a laser light source in a deep ultraviolet region using a nonlinear crystal.
2. Description of the Background
Generally, a laser beam has characteristics that since the frequency is higher than that of an electric wave, information storage capacity is large; furthermore, since the wavelength is equal and the phase is uniform, a laser beam is excellent in monochromaticity and directivity and has coherency which is not seen for a normal beam; further, since a laser beam can be extremely narrowly converged, energy is focused on a minute area to realize high temperature and high voltage locally and instantaneously, resulting in application to many fields such as communications and information fields, measurement field, application to process technology, and application to medical field.
In the laser usage where a short wavelength is required, a laser in a deep ultraviolet region is used. However, since there are limitations of a laser medium and an excitation source in the deep ultraviolet region, laser oscillation is difficult. Consequently, as for a laser light source in the deep ultraviolet region, there are many methods which perform wavelength conversion of laser output of visible region from relatively stable near-infrared, in particular a method which uses harmonic generation and sum frequency generation using a nonlinear crystal.
In the case of the harmonic generation, laser output of integral multiple frequency such as frequencies 2ω and 3ω, that is, laser output of a wavelength which becomes short to λ/2 and λ/3, with respect to an input wavelength λ, can be obtained by inputting a laser beam of a frequency ω to a nonlinear optical crystal.
Furthermore, in the case of the sum frequency generation, laser output whose frequency is cω (=ω1+ω2), that is, laser output of λ3 whose wavelength is shorter than that of either input wavelength λ1 or λ2 that fulfills a relationship, 1/λ3=1/λ1+1/λ2, with respect to input wavelengths λ1 and λ2, can be obtained by inputting a laser beam of two different frequencies ω1 and ω2 to the nonlinear crystal.
Naturally, BBO crystal (BaB2O4) and CLBO crystal (CsLiB6O10), which are nonlinear crystals for use in sum frequency generation and harmonic generation in a deep ultraviolet region, are subject to damages due to light or circumference moisture. Then, damages produced on the nonlinear crystal cause degradation of laser beam output and change in intensity distribution. So, there arises a problem in that measurement accuracy and process quality deteriorate in the inspection, measurement, and process with laser beam.
Consequently, in the case where a nonlinear crystal is generally operated for a long time, a crystal with a certain degree of size is used; and, when damage is produced, the nonlinear crystal is moved so that a non-damaged part is used. Therefore, the nonlinear crystal is installed on a micromotion mechanism. Furthermore, the nonlinear crystal is installed in a space so as to purge circumferential atmosphere and is heated, so as to be at constant temperature, with a heater or the like for preventing it from being damaged due to moisture.
As described above, in the method in which the nonlinear crystal is moved each time the nonlinear crystal is damaged, it is indispensable to judge whether or not damage exists.
For example, an operation method, in which a constant time is set based on the evaluation result separately carried out and the crystal is moved when an operating time of a laser light source exceeds the predetermined time, is adopted. However, since this method generally sets a sufficient margin for damage, the crystal is actually moved regardless of no degradation of laser output; therefore, there is a problem in that operating life of an expensive nonlinear crystal cannot be effectively used. Furthermore, as a result, there arises a problem in that effective use of a laser device cannot be made because maintenance frequency of the laser device increases.
Consequently, in a laser device using a nonlinear crystal, a method, in which a part of laser beam irradiating a workpiece is separated and the separated laser beam is observed to monitor damage of the nonlinear crystal, is disclosed in a document (for example, refer to Japanese Patent Application, Publication No. 2003-46173). FIG. 8 is a configuration view showing a laser device of a prior art. In FIG. 8, a laser beam is output from a laser oscillator 82 by electric power supplied by a laser power supply 89. Then, the laser beam output from the laser oscillator is split by a beam splitter 88. Then, intensity of one laser beam of the split laser beams is measured by a light sensitive sensor 84. Degradation of the laser beam intensity, that is, the presence or absence of damage of the nonlinear crystal is judged by monitoring change in this laser beam intensity.
For a certainty, according to this prior art, damage of the nonlinear crystal for use in the laser oscillator can be monitored by monitoring change in the laser beam output simultaneously with laser processing. However, a scattered light which is generated when the nonlinear crystal is damaged is certainly superimposed on the laser beam output. Therefore, there is a problem in that if degradation of the laser beam due to damage of the nonlinear crystal is equivalent to the order of increase in scattered light, both are offset, so that it becomes truly difficult to judge the presence or absence of damage of the nonlinear crystal.